In beams of polarized electrons, the spins of the electrons are predominantly oriented in an optional fixed direction in space. The degree of spin polarization P is defined by the equation P = (n.uparw. - n.dwnarw.) / (n.uparw. + n.dwnarw.) where n.uparw.(n.dwnarw.) is the number of electrons with spin parallel (antiparallel) to the direction in space.
Electron beams are commonly used in science and technology as a diagnostic tool to elucidate even the smallest structures and to display pictures and information on phosphor screens or similar elements. To date unpolarized electron beams have been used almost exclusively for this purpose simply because no available sources of polarized electrons with a high degree of polarization and high intensity were known. The invention provides a simple and intensive source of highly polarized electrons through which now contrast can be obtained in the study of structures by means of the interaction of the spin with the object of investigation. The capability of the invention to regulate the preferred orientation of the spin provides a new degree of freedom which can now also be used in information transfer.
Different types of sources of polarized electrons are already known. The most intensive sources use a ferromagnet in some form, from which electrons are emitted into vacuum by photoemission of field emission. The electrons are spin polarized if the ferromagnet is cooled to a temperature below the Curie temperature and if at the same time the magnetic domains are aligned by applying a magnetic field.
The disadvantage of these known sources is that a magnetic field must be applied at the source. The magnetic field has the following disadvantageous electron -optical effects.
1. The electrons can be extracted from the magnetic field and formed into a beam only with a loss of intensity.
2. A certain minimum energy of the electrons is required. However, for many applications one needs very low energy electrons.
3. The spin direction of the electrons is reversed by reversing the magnetic field. However, the speed with which a magnetic field can be reversed is limited by the law of induction. On reversing the magnetic field no electron optical effects, such as a shift of the electron beam, can be tolerated. This requires a practically unrealizable precision on the adjustment of the source and the electron optical axis with the axis of the magnetic field.
The object of the invention is therefore to make a source of polarized electrons which does not require a magnetic field.